Almost all of the tournaments I choose to play have a guaranteed prize pool but often, the number of buy-ins exceeds the guarantee and then it becomes a standard prize pool equal to the number of buy-ins.

The catch 22 here is that without the guarantee, it may not attract enough players to exceed the guaranteed prize pool.

Once it has exceeded the guarantee I essentially lose any value I gained by entering a tournament that had a greater ROI for my buy-in than it should.

In very rare cases, the buy-ins do not exceed the guarantee, and then I have extra value and less opponents to beat.

Why do poker sites offer guaranteed prize pools?

I assume it's because they know/hope that it will be exceeded as well as attracting more people, so ultimately more money for them.

How can I assess the value it provides to me when the number of buy-ins does not exceed the guarantee?

Well, the reason that you want to play in a guarantee even when there are more people than required in order to make the guarantee is that you are able to play in a tournament with enough entries to have a prize pool worth playing. Yes, if they didn't make the guarantee it is even better, but if they didn't offer it, instead of being guaranteed a, say, 10K prize pool, you may only be playing against enough people to make a $3K prize pool without it being worth your time to play at all.
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lnafzigerSep 3 '13 at 22:29